Tryptophan synthase of Escherichia coli is a multienzyme complex composed of 2 alpha subunits and 1 Beta 2 subunit. The Beta 2 subunit contains 2 moles of pyridoxal-P and catalyzes a number of pyridoxal-P dependent reactions. The mechanism of the Beta 2 subunit and its regulation by interaction with 2 alpha subunits are being investigated in order to understand more about enzyme interaction and regulation in multienzyme systems. Reassociation of the alpha and Beta 2 subunits to form an alpha 2 Beta 2 complex has been shown to result in changes in the environment of aromatic amino acids. Difference absorbance spectra, circular dichroism spectra, and chemical modification studies have been used. About 6 tyrosyl residues are transferred from an exposed environment to a hydrophobic environment when the alpha 2 Beta complex is formed. Derivatives of the Beta 2 subunit and the alpha 2 Beta 2 complex formed by reaction with substrate analogs or reagents which react with pyridoxal-P are being compared. All derivatives of the alpha 2 Beta 2 complex are much more stable than those of the Beta 2 subunit. These experiments give comparative information on the environments of the active site of the Beta 2 subunit in the presence and absence of the alpha subunit.